It is known that silicon nitride ceramics having acceptably high strength can be obtained by a reaction bonding process but that such processes have the disadvantage of being very time-consuming, requiring, e.g., 72 hours or more for completion. As disclosed in U.S. Pat. Nos. 4,543,344 (Cannady) and 4,612,383 (Laine et al.), it is also known that silicon nitride ceramics can be derived from polysilazane precursors. However, the strengths of silicon nitride ceramics derived from mixtures of silicon nitride powders and polysilazane precursors have hitherto been quite poor. To overcome the disadvantages of these known methods of preparing silicon nitride ceramics, it would be desirable to find a way of deriving silicon nitride ceramics from mixtures of silicon nitride powders and polysilazane precursors that would result in the ceramics' having high strength, i.e., a strength higher than that of the known silicon nitride ceramics derived from silicon nitride powders and polysilazane precursors, e.g., a bending strength of at least about 23 kg/mm.sup.2 and a specific strength of at least about 10 (kg/mm.sup.2)/(g/cc).
U.S. Pat. Nos. 4,482,669 (Seyferth et al.-I), 4,645,807 (Seyferth et al.-II), 4,650,837 (Seyferth et al.-III), and 4,659,850 (Arai et al.) disclose the utility of polysilazanes as preceramic materials, and Seyferth et al.-I teach that their polysilazanes are especially useful as binders for ceramic powders such as silicon nitride. Silicon nitride ceramics prepared from these silicon nitride powder/polysilazane binder compositions are composed predominantly of silicon nitride, e.g., about 92% silicon nitride or more, with the balance being mostly silicon carbide. Seyferth et al.-I do not disclose the typical densities and bending strengths of ceramics made from their preceramic compositions. However, Wiseman, "The Development and Application of Polysilazane Precursors to Ceramics," a Massachusetts Institute of Technology thesis, 1984, shows that these densities were generally about 2.1-2.2 g/cc and the bending strength were poor. Wiseman showsa recognition, though, of its being desirable to minimize alkali metal contamination and to use preceramic polymers having a sufficiently high molecular weight, or mixtures (such as 80/20 mixtures) of such polymers with lower molecular weight polymers, to maximize strength.